1. Field of the Invention
This invention relates to projection lens systems for use in projection televisions and, in particular, to color-corrected projection lens systems which employ both glass and plastic lens elements, i.e., lens systems of the hybrid type.
2. Description of the Prior Art
Projection lens systems for CRT projection televisions have undergone continuing development during the past fifteen years or so. As a result, many of today's CRT projection televisions are equipped with fast lens systems which provide wide semi-fields of view.
When the field of view does not exceed a half-angle of about 25.degree., the lens system may consist of just three components and still provide a sufficiently high level of image quality. A typical configuration has a weak aspherical element on the image side of the lens system, followed by a strong positive power element, followed in turn by a strong negative element in close proximity to the CRT. See, for example, Betensky, U.S. Pat. Nos. 4,300,817, 4,348,081, and 4,526,442.
For this configuration, the aspherical first element provides correction primarily for aperture dependent aberrations including spherical aberration, the positive element provides most of the optical power of the system, the relative position of the aperture stop provides correction for astigmatism and lateral color, and the negative element close to the CRT provides correction for field dependent aberrations including the field curvature of the lens.
More generally, lens systems of this type can include more than three elements with the elements being arranged in three lens units, i.e., a "first" or "A" lens unit located on the image side of the lens system of weak power, a "second" or "B" lens unit following the first lens unit of strong positive power, and a "third" or "C" lens unit following the second lens unit of strong negative power.
When the focal length of the lens system is shortened to make the projection television more compact in the case of rear projection televisions or to make the magnification larger in the case of front projection televisions, the field coverage of the lens system must be increased. When the half-angle of view is increased to about 28.degree., a three element form generally cannot provide an appropriately high level of optical performance.
To address this problem, a fourth element or more generally a fourth unit (hereinafter referred to as the "corrector" or "CR" unit) has been added between the strong positive and the strong negative units of the three unit configuration, i.e., between the B and C units. See Betensky, U.S. Pat. No. 4,697,892, and Moskovich, U.S. Pat. Nos. 4,682,862, 4,755,028, and 4,776,681. This additional unit usually does not have much optical power; however, it must have an aspherical surface to correct for aperture dependent off-axis aberrations like sagittal oblique spherical and coma.
Color images for projection televisions are normally obtained by combining images from three color CRTs, i.e., a red CRT, a green CRT, and a blue CRT. The phosphors used in commercially available CRTs do not emit light at a single wavelength. In particular, green phosphors have significant sidebands in blue an red. Similar polychromaticity exists for red and blue phosphors, but to a lesser extent.
For many consumer applications, lens systems uncorrected for color can be used, notwithstanding the color spread of the CRTs. For more demanding applications, however, such as high definition television, data displays, or systems which operate at a high magnification, color correction is needed to avoid visible color fringing and/or a loss of image contrast.
A color-corrected projection lens is disclosed in Kreitzer, U.S. Pat. No. 4,900,139. This lens system employs the A, B, C, and CR units discussed above and achieves longitudinal color correction by means of, inter alia, a color-correcting doublet which is included in the B unit. Although lens systems constructed in accordance with this patent have worked well in practice, these systems can be susceptible to focus shifts with changes in temperature as a result of the use of plastic lens elements having not insignificant optical powers in the A unit. Also, the A unit plastic elements have had relatively non-uniform thicknesses making them relatively difficult to fabricate using injection molding techniques. Moreover, strong aspherical plastic elements in the A unit have high tolerance sensitivity.
The use of a plastic element having a relatively constant thickness to form a CR unit is discussed in the Moskovich '862 patent referred to above (see column 13, line 67, to column 14, line 28). The lens systems described in the examples of this patent are not color corrected. Also, the A units of those examples employ plastic elements having not insignificant optical powers, i.e., the A units may be subject to relatively large changes in their power as a result of changes in temperature.